1. Field of the Invention
The present invention generally relates to image forming apparatuses and, more particularly, to an image forming apparatus using electrophotography such as a laser printer equipped with an optical scanner using a laser, a digital copy machine or a facsimile machine.
2. Description of the Related Art
Conventionally, there is a laser beam method as one of methods with which an image forming apparatus forms a visible image. According to the laser beam method used in an image forming apparatus, an optical scanning apparatus irradiates a laser beam onto a photoconductor (a medium to be scanned) so as to form an image.
A description will be given below, with reference to FIG. 1, of an image forming operation performed by an image forming apparatus using a conventional laser beam method. FIG. 1 is an illustration for explaining an image forming operation performed by an image forming apparatus using a conventional laser beam method.
In the image forming apparatus, an electric charger (not shown in the figure) such as a roller-type contact charger uniformly charges a photoconductor. A laser light source irradiates a laser beam onto a rotating polygon mirror (rotational polygon mirror). The polygon mirror deflects periodically the laser beam projected from the laser light source. The laser beam passes through a fθ lens and scans a surface of the photoconductor in a main scanning direction while the photoconductor is moving or rotating in a sub-scanning direction. Static electricity in portions of the surface of the photoconductor onto which the laser beam is irradiated is removed by the laser beam, thereby forming a pattern of static electricity (electrostatic latent image) on the surface of the photoconductor.
It should be noted that a controller (not shown in the figure) sends image data corresponding to each page to a laser drive circuit as an image signal (video signal) on an individual line (one scanning line) basis. Then, the laser drive circuit performs a modulating operation by outputting the image signal to the laser light beam in synchronization with a pixel clock (write clock). The pixel clock is supplied to the laser drive circuit from a pixel clock generating circuit (not shown in the figure) via a phase synchronization circuit (not shown in the figure). The pixel clock generating circuit and the phase synchronization signal constitute image clock generating means and phase setting means.
A description will now be given, with reference to FIG. 2, of a relationship between the pixel clock and the phase change (phase setting) of the pixel clock. FIG. 2 is a timing chart showing a relationship between a conventional pixel clock and a phase change thereof.
A pixel clock generating circuit generates and outputs a pixel clock clkw in synchronization with a synchronization detection signal supplied by a synchronous detection sensor. The pixel clock clkw is generated from a reference clock clko (an original clock generated by an oscillator (not shown in the figure)), which is n times (four times in FIG. 2) of the frequency of the pixel clock clkw, by toggling the reference clock clko to a high level (H) and a low level (L) at each 4 pulses of clko according to a counting control. In the above-mentioned optical scanning apparatus, when forming beam spots of the laser beam on the surface to be scanned so as to write an electrostatic latent image, the write density of the beam spots is adjusted to be uniform.
However, if an environmental fluctuation such as a temperature change, etc. arises in the atmosphere of the fθ lens, the fθ lens is distorted, which results in a change in an index of refraction. Additionally, if an environmental fluctuation such as a temperature change, etc. arises in the atmosphere of the laser light source, the wavelength of the laser beam emitted by the laser light source is changed. Therefore, the fθ lens is configured and arranged to refract the laser beam at a predetermined angle according to the wavelength of the laser beam incident thereon. For this reason, as shown in FIG. 1, an error occurs in the refractive angle of the laser beam incident on the fθ lens. Such an error in the refractive angle causes an error in a write magnification per one main scanning period of the laser beam by the polygon mirror, which may give an undesired influence to an output image. In order to eliminate such an undesired influence, a phase change to shift the phase of the pixel clock clkw is performed so as to correct the write magnification of the laser beam.
In the above-mentioned optical scanning apparatus, the pixel clock generating circuit performs a phase control using an external pulse train xpls so as to perform the phase change to shift the phase of the pixel clock clkw. There are two kinds of pulse train as the external pulse train xplsp, one is an external pulse train xplsp for advancing the phase of the pixel clock clkw and the other is an external pulse train xplsm for delaying the phase of the pixel clock clkw. For example, in a case in which the pixel clock clkw is generated from the reference clock clko, the pixel clock clkw, which is usually generated with 8 pulses of clko, is generated with 9 pulses of clko or 7 pulses of clko by increasing or decreasing the number of count of the pulses of the external pulse train xplsp or xplsm. By changing the number of counts by increasing or decreasing the number of counts, the frequency of the pixel clock clkw is made 8/7 times (an advance control) or 8/9 times (a delay control), which can shift the pixel clock clkw after the phase change. This provides an effect in view of one main scanning line that an entire magnification is increased or decreased as Tm-7/8 (advancing control) or Tm+9/8 (delay control), where Tm is a total time of main scanning of one line. Thus, the optical scanning apparatus can form an image on a desired position on the photoconductor without being influenced by the environment fluctuation.
Moreover, the pixel clock generating circuit is provided with a pulse generating circuit which generates the above-mentioned external pulse train xpls. The pulse generating circuit generates the external pulse train (hereinafter, may be simply referred to as pulses) xpls in response to positions at which the phase change is to be applied to the pulse train of the pixel clock clkw.
A description will now be given, with reference to FIG. 3, of an operation of generating the external pulse train by the pulse generating circuit. FIG. 3 is a block diagram of a conventional pulse generating circuit.
The pulse generating circuit shown in FIG. 3 comprises comparators 1001 and 1002 and counters 1003 and 1004.
In the pulse generating circuit, a pulse generation interval (period) is set to the comparator 1001 and a pulse number num is set to the comparator 1002 by an engine CPU (not shown in the figure) so as to perform the following operation when one scan of a laser beam is performed in the main scanning direction by the polygon mirror.
The counter 1003 starts a counting operation to count a number of pulses of the pixel clocks clkw (count value i) in accordance with an input of a clear signal xlclr generated from a synchronization detection signal by a circuit, which is not shown in the figure, at a time of input of the clear signal xlclr as a reference, and stops the counting operation when a stop signal is supplied by the comparator 1002. The comparator 1001 compares the count value i of the counter 1003 with the previously set pulse generation interval (hereinafter, may be referred to as a setting value) prd, and generates the pulse xpls each time the count value i reaches the setting value prd. The counter 1004 counts the number (count value j) of the pulses xpls generated by the comparator 1001. The comparator 1002 compares the count value j of the counter 1004 with the previously set pulse number (hereinafter, may be referred to as a setting value) num, and generates the stop signal when the count value j reaches the setting value num.
FIG. 4 is a flowchart of an operation of the pulse generating circuit shown in FIG. 3. A description will now be given, with reference 3 to FIGS. 3 and 4, of an operation of generating the external pulse train xpls performed by the conventional pulse generating circuit.
First, counters 1003 and 1004 reset count values i and j to “1”, respectively, when a power supply to the pulse generating circuit is turned on (step S1001).
Thereafter, the counter 1003 waits for an input of a clear signal xlclr (No of step S1002), and after the clear signal xlclr is input (Yes of step S1002), the counter 1003 counts up (+1) the count value i each time a pulse of the pixel clock clkw is input.
A comparator 1001 performs the process of step S1003 until the count value i reaches the setting value prd (No of step S1004).
The comparator 1001 will generate the pulse xpls, when the count value i reaches the setting value prd (Yes of step S1004). The counter 1003 returns the count value i to “1” according to the input of the pulse xpls (step S1005).
Additionally, as a result of the comparison between the count value j of the counter 1004 and the setting value num by the comparator 1002 (step S1006), if the count value j has not reached the setting value num (No of step S1006), the counter 1004 counts up (+1) the count value j according to the input of the pulse xpls (step S1007).
Thereafter, the counters 1003 and 1004 and the comparator 1001 repeat the above-mentioned operation, and the comparator 1002 generates a stop signal when the count value j reaches the setting value num. Thus, the pulse generating circuit ends the operation (hereinafter, this operation is referred to as “pulse generating operation”).
Here, Japanese Patent No. 3315474 discloses a conventional technique related with a phase control of the pixel clock in an image forming apparatus. In the image forming apparatus disclosed in this patent document, there are provided a plurality of detection sensors that detect a laser beam moving in a main-scanning direction, and a scanning time or a count number of the clock is measured from a time when the laser beam is detected by one of the detection sensors and until the laser beam is detected by another one of the detection sensors so as to correct a write clock frequency in accordance with the result of the measurement.
Additionally, there is a multi-beam method for forming an image on a photoconductor using more than two laser light sources among laser beam methods for forming an image by irradiating a laser beam onto a scan surface of a photoconductor.
The image forming apparatus using the multi-beam method increases a speed of forming a latent image by writing beam spots on more than two main-scanning lines simultaneously.
FIG. 5 is an illustration showing an image forming operation performed by a conventional image forming apparatus using the multi-beam method. FIG. 5 shows an example in which two laser beams are irradiated by two laser light sources LD0 and LD1. As shown in FIG. 5, the two laser light sources LD0 and LD1 irradiate laser beams onto a rotating polygon mirror. The polygon mirror deflects periodically the laser beams projected from the laser light sources LD0 and LD1. The deflected laser beams transmit through a fθ lens and repeatedly scan a surface of a photoconductor to be scanned that is rotating in a sub-scanning direction while uniformly charging the surface of the photoconductor. Portions irradiated by the laser beams on the photoconductor are electrically discharged, thereby forming an electrostatic latent image.
Japanese Laid-Open Patent Application No. 2000-166598 discloses a multi-beam light source apparatus and an optical scan apparatus using the above-mentioned multi-beam method. In this patent document, there are provided two light source parts each of which has two light source elements, and an offset of projecting axes of the two light source parts is corrected by adjusting a relative angle of light beams projected from the two light sources.
However, there are the following problems in the image forming apparatus using the multi-beam method in which a plurality of laser light sources shares a single optical system and a single image carrier.
As mentioned above, a fluctuation occurs in the refraction index of a group of lenses including the fθ lens and the wavelength of the laser beams projected from the laser light sources. Therefore, an error occurs in the write magnification of each of the laser beams projected from the plurality of laser light beams per one scanning period, and, thus, there is a problem in that positions of the images written by the laser light beams are shifted from each other. Consequently, a line extending in the sub-scanning direction appears as it wobbles and a noise is generated in an entire image, and, thus, there is a problem in that an unclear image is formed.
Additionally, the group of lenses including the fθ lens forming an optical system are made in consideration of refraction of a specific wavelength. Therefore, if a plurality of laser light sources share a single optical system and a single image carrier in the image forming apparatus using the multi-beam method, a plurality of laser beams that emit laser beams having the same wavelength must be selected and provided in the image forming apparatus.
However, it is difficult to prepare a certain number of laser light sources that emit laser beams having accurately the same wavelength, and, thus, there is a problem in that the formed image is unclear.